The present invention relates to a switch assemblies in general and particularly to membrane type switches.
Membrane switches are used in many electronic devices such as calculators, appliance control panels, automotive dashboards, consumer toys, and industrial controls. These type of switches enjoy several advantages over conventional rocker, toggle or key switches. They can provide a liquid tight front surface, making them quite useful in environments where a switch or switch assembly might be exposed to liquid spills, corrosive chemicals, or moisture. They offer a reduced thickness, due to the absence of mechanical parts such as springs, rockers, wiring fasteners, and large housings. This feature is extremely desirable in applications such as thin calculators and other portable consumer electronic devices that must be worn on the person or carried in a pocket. Because of the absence of such parts, they have a lower cost of materials and assembly. This factor is quite important in consumer electronics. They provide a momentary contact, as opposed to permanent contact switch. This is important in applications such as calculators where a circuit is meant to be only briefly actuated.
FIG. 1 shows a cross sectional view of a typical membrane switch. In principal, a membrane switch is constructed much like a conventional switch, that is a movable contact is used to momentarily complete the connection between two sides of a circuit that are normally not connected. In membrane switches, the normally nonconnected sides of the circuit are formed on the surface of a rigid or flexible printed circuit board 10 that usually contains other circuitry 11 to interconnect the switch or switches to the remainder of the electronic device. The movable contact consists of a flexible plastic sheet 12 which has a conductive area 13 that serves to complete the circuit on the printed circuit board when it is moved into contact with it. The conductive area 13 on the flexible sheet is arranged so as to be facing the circuit board 10 and is aligned so as to be directly opposite the stationary contacts 14 on the circuit board. The flexible sheet 12 is bonded with an adhesive 15 to a spacer 16, which serves to maintain a fixed distance between the circuit board traces 14 and the movable contact 13. The spacer 16 is typically 2-10 mils thick, and is bonded using adhesive 15 to the circuit board 10. When a force is applied to the exterior side of the flexible sheet 12, the movable contact 13 is displaced into an opening in the spacer 16 and into engagement with the stationary contact 14 to close the switch.
This assembly is usually placed into a housing 17 or else it is bonded to another rigid member to provide rigidity and mechanical strength. FIG. 2 shows a cross sectional view of a membrane switch boonded to a metal plate 18. In order to prevent the circuit traces on the bottom side of the printed circuit board 10 from shorting to the metal plate 18, an insulator 19 is placed between the printed circuit board and the metal plate. This insulator 19 is typically bonded to the circuit board and the metal plate with a layer of adhesive 20 at each interface. The membrane switch assembly must be rigid enough to withstand any deflection incurred by the actuation force on the flexible sheet 12, or the entire assembly will deflect and the switch will not function. In addition, the spacer thickness and the size of the aperture in the spacer must be carefully controlled in order to maintain uniform actuation force on the switch. Misalignment or damage to the spacer during assembly is common and is cause for the switch to malfunction.